I think the more appropriate question might be how did I discover what the problem was in the first place .
I first noticed that the 8 year old Fridigaire was not running. I unplugged and pluged it in and heard a click sound down in the back. So I put my hand on a couple of components and felt the defrost timer vibrate.
Now here is where the luck came in to prove my theory. I had a matching freezer of the same year to go with this Frig that stopped working. Since it was not in use at the time I took out the identical defrost timer and put it in the frig and what do you know it's back working. The first time some years ago when it was still under warranty the 800# service tech person helped me replace the thermostat in the upper section of the unit. When you live so far from service people you dig in and figure it out.